1. Field of the Invention
The present invention relates to a process for lessening residual halogen content in alkoxysilanes or alkoxysilane-based compositions. More particularly, the present invention relates to improving the color number of alkoxysilanes or alkoxysilane-based compositions.
2. Description of the Background
Alkoxysilanes and alkoxysilane-based compositions are employed in numerous fields: as adhesion promoters, as release agents, as crosslinkers in polymers, as additives in paints and coating materials, for the hydrophobicization of surfaces, inter alia for textiles and leather and, in particular, for the protection of buildings and facades, for book preservation, for the special modification of the properties of surfaces such as the coating of glass fibers or the silanization of fillers and pigments, and also for improving the rheological properties of polymer dispersions and emulsions, to name but a few examples.
Alkoxysilanes are normally prepared from halosilanes, especially chlorosilanes. For example, by reacting an alkyltrichlorosilane with methanol an alkyltrimethoxysilane is obtained and HCl is evolved. Even after distillative purification of the product, residual amounts of acid chloride may remain in the alkoxysilane in the order of magnitude of from around 50 to 500 ppm by weight. In the case of the preparation of, for example, aminoalkyltrialkoxysilanes by reacting chloroalkyltrialkoxysilanes with NH.sub.3 or an alkylamine and then separating the ammonium chloride salt produced, which is the predominant chloride product, by filtration, residual amounts of acidic or hydrolyzable chloride remain in the product. In addition, the product may also contain residual amounts of what is known as nonhydrolyzable chlorine, for example, unreacted starting materials such as chloroalkylalkoxysilanes.
Currently, alkoxysilanes, especially oligomeric alkoxysilanes or silane systems based thereon, and products which comprise alkoxysilanes, cf. e.g. EP O 273 867 B1, DE 27 51 714 C2, and alkoxysilane-based compositions, as disclosed, for example, by EP O 049 365 A2, EP O518 057 A1, EP O 675 128 A1, EP O 716 127 A2or EP O 716 128 A2, to name but a few, are prepared with the objective of having very low halide contents, especially of acidic or hydrolyzable chlorides.
It is known that the residual halogen content can be removed from alkoxysilanes by means, inter alia, of reaction or neutralization with alkali metal alcoholates such as sodium methanolate, and separation of the resultant salt (EP O 282 486 A2, EP O 741 137 A1). Neutralization methods of this kind are usually laborious and in general cannot be applied to stabilized alkoxysilane-based compositions.
It is also known to employ activated carbon as an adsorbent for removing, inter alia, chlorine from water (Rompp Chemie Lexikon, 9th edition, page 83, under Activated carbon and page 59 under Adsorbents).
It is also known, as disclosed in Chemical Abstracts Vol. 117 (1992), p. 713, CA 117: 251554n),to remove chloride compounds from alkyl- and arylalkoxysilanes by treatment with an alkali-treated activated carbon and basic ion exchange resins.
DE-A 28 07 589 discloses the removal of biphenyls and other impurities from impure silanes and siloxanes by bringing them into contact with a fixed bed of molecular sieves and carbon.
U.S. Pat. No. 5,567,836 discloses a process in which methyltrichlorosilane is separated from dimethyldichlorosilane using activated carbon.
A need continues to exist for improvements in the preparation of high purity alkoxysilanes.